Fixed NPE when setting selector explicitly to null from groovy

[cconnert]

When doing e.g.

def selector = someCondition ? specific(BUILD)) : null;
copyArtifacts(fingerprintArtifacts: true, selector: selector, projectName: "someProject")

leads to:

Also:   org.jenkinsci.plugins.workflow.actions.ErrorAction$ErrorId: xyz
java.lang.NullPointerException
	at hudson.plugins.copyartifact.CopyArtifact.perform(CopyArtifact.java:482)
	at jenkins.tasks.SimpleBuildStep.perform(SimpleBuildStep.java:123)
	at org.jenkinsci.plugins.workflow.steps.CoreStep$Execution.run(CoreStep.java:101)
	at org.jenkinsci.plugins.workflow.steps.CoreStep$Execution.run(CoreStep.java:71)
	at org.jenkinsci.plugins.workflow.steps.SynchronousNonBlockingStepExecution.lambda$start$0(SynchronousNonBlockingStepExecution.java:47)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:834)

though I would expect to get the default (aka. stable) artifact selector.

Testing done

No, please add a dedicated unit test

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